Some air-conditioning apparatuses, such as multi-air-conditioning apparatuses for buildings, have a heat source device (outdoor unit) disposed outside the building, and an indoor unit disposed within a room in the building. A refrigerant circulating within a refrigerant circuit of the air-conditioning apparatus transfers heat to (absorbs heat from) air supplied to a heat exchanger of the indoor unit to heat or cool this air. The heated or cooled air is transported into a space to be air-conditioned to perform heating or cooling.
Such an air-conditioning apparatus is constituted by multiple indoor units since buildings generally have multiple indoor spaces. Also, in the case where the scale of a building is great, a refrigerant pipe which connects an outdoor unit and indoor units may have a length of 100 m. The longer the length of the pipe which connects the outdoor unit and the indoor units, the more the refrigerant fills the refrigerant circuit.
The indoor units of such a multi-air-conditioning apparatus for buildings are generally disposed in indoor spaces where people are present (for example, office spaces, living rooms, shops, etc.). In the case where a refrigerant has leaked from an indoor unit disposed in an indoor space for one reason or another, there may occur a problem in terms of the influence on the human body and safety because some refrigerant might be flammable or toxic depending on its type. Also, even if this refrigerant is a refrigerant which is not harmful to the human body, it may be anticipated that the oxygen concentration in the indoor space will decrease due to refrigerant leakage, which can affect the human body.
To address this problem, there is a method in which with the use of the secondary loop method, a refrigerant is circulated in the primary side loop and a harmless heat medium such as water or brine is circulated in the secondary side loop, so that the heating energy or cooling energy of the refrigerant is transmitted to the heat medium (for example, see Patent Literature 1). In the technology disclosed in Patent Literature 1, the heating energy or cooling energy generated in the primary side loop is transmitted to the secondary side loop via an intermediate heat exchanger such as a plate heat exchanger or a double pipe, and the heating energy or cooling energy is supplied to an indoor unit through the secondary side loop. Also, in the technology disclosed in Patent Literature 1, a pipe corresponding to the secondary side loop where this harmless heat medium circulates is situated near a space where a person is present, and accordingly, the effect on the human body due to refrigerant leakage can be suppressed.
On the other hand, when an outdoor unit heat exchanger which executes a heating operation serves as an evaporator, frost is apt to be formed on fins of the outdoor unit heat exchanger where the temperature of the outside air is low. The formation of frost disturbs heat exchange between the outdoor air supplied by a fan or the like and the refrigerant flowing through a tube of the outdoor unit heat exchanger, resulting in a reduction in heat exchange efficiency. There is known a technology in which to remove the frost of the outdoor unit heat exchanger, a defrost operation is performed to supply a high-temperature refrigerant to the outdoor unit heat exchanger.
When this defrost operation is applied to the technology disclosed in Patent Literature 1, the refrigerant flowing into the intermediate heat exchanger via the outdoor unit heat exchanger and an expansion device, and flowing out from the intermediate heat exchanger is sucked into the compressor again. Here, the refrigerant flowing out from the expansion device has a low temperature due to operating as a condenser of the outdoor unit heat exchanger, and the pressure of the refrigerant is reduced due to the operation of the expansion device.
Thus, the refrigerant flowing out from the expansion device may receive heat from the heat medium in the secondary side loop and evaporate in the intermediate heat exchanger, thereby freezing this heat medium. Therefore, there has been proposed an air-conditioning apparatus including a bypass pipe which bypasses an intermediate heat exchanger (for example, see Patent Literature 2). The technology disclosed in Patent Literature 2 suppresses freezing of the heat medium in the secondary side loop by making the flow resistance of the bypass pipe smaller than that of the intermediate heat exchanger and reducing the amount of the refrigerant flowing into the intermediate heat exchanger.